A further model for temporal patterns in the epidemiology of schistosome infections of snails

Land use/land cover change, physico-chemical parameters and freshwater snails in Yewa North, Southwestern Nigeria

The management of ecosystem has been a major contributor to the control of diseases that are transmitted by snail intermediate hosts. The ability of freshwater snails to self-fertilize, giving rise to thousands of hatchlings, enables them to contribute immensely to the difficulty in reducing the endemicity of some infections in the world. One of the effects of land use/land cover change (LU/LCC) is deforestation, which, in turn, leads to the creation of suitable habitats for the survival of freshwater snails. This study was aimed at studying the land use/land cover change, physico-chemical parameters of water bodies and to understand the interplay between them and freshwater snails in an environment where a new industrial plant was established. Landsat TM, 1984, Landsat ETM+ 2000 and Operational land Imager (OLI) 2014 imageries of the study area were digitally processed using ERDAS Imagine. The land use classification includes settlement, water bodies, wetlands, vegetation and exposed surface. Dissolved oxygen, water temperature, pH, total dissolved solids and conductivity were measured with multipurpose digital meters. Snail sampling was done at each site for 30 minutes along the littoral zones, using a long-handled scoop (0.2mm mesh size) net once every month for 24 months. Independent t-test was used to determine the variation between seasons, Spearman’s rank correlation coefficient was used to test the relationship between physico-chemical parameters and snail species while regression was used to analyze the relationship between LU/LCC and freshwater snails. Species’ richness, diversity and evenness were examined using Margalef, Shannon Weiner and Equitability indexes. Snail species recovered include: Bulinus globosus, Bulinus jousseaumei, Bulinus camerunensis, Bulinus senegalensis, Bulinus forskalii, Amerianna carinatus, Ferrissia spp., Segmentorbis augustus, Lymnaea natalensis, Melanoides tuberculata, Physa acuta, Gyraulus costulatus, Indoplanorbis exuxtus and Gibbiella species. Out of the total snails recovered, M. tuberculata (2907) was the most abundant, followed by Lymnaea natalensis (1542). The highest number of snail species was recovered from Iho River while the least number of snails was recovered from Euro River. The mean and standard deviation of physico-chemical parameters of the water bodies were DO (2.13±0.9 mg/L), pH (6.80±0.4), TDS (50.58±18.8 mg/L), Temperature (26.2±0.9°C) and Conductivity (74.00±27.5 μS/cm). There was significant positive correlation between pH and B. globosus (r = 0.439; P<0.05). Dissolved oxygen showed significant positive correlation with B. globosus (r = 0.454; P<0.05) and M. tuberculata (r = 0.687; P<0.01). There was a positive significant relationship between LULCC and B. camerunensis (p<0.05). The positive relationship between LULCC and the abundance of B. globosus, B. jousseaumei was not significant. The area covered by water bodies increased from 3.72 to 4.51 kilometers; this indicates that, more suitable habitats were being created for the multiplication of freshwater snails. We therefore conclude that, increase in areas suitable for the survival of freshwater snails could lead to an increase in water-borne diseases caused by the availability of snail intermediate hosts.

Towards interruption of schistosomiasis transmission in sub-Saharan Africa: developing an appropriate environmental surveillance framework to guide and to support 'end game' interventions

Schistosomiasis is a waterborne parasitic disease in sub-Saharan Africa, particularly common in rural populations living in impoverished conditions. With the scale-up of preventive chemotherapy, national campaigns will transition from morbidity- to transmission-focused interventions thus formal investigation of actual or expected declines in environmental transmission is needed as 'end game' scenarios arise. Surprisingly, there are no international or national guidelines to do so in sub-Saharan Africa. Our article therefore provides an introduction to key practicalities and pitfalls in the development of an appropriate environmental surveillance framework. In this context, we discuss how strategies need to be adapted and tailored to the local level to better guide and support future interventions through this transition. As detection of egg-patent infection in people becomes rare, careful sampling of schistosome larvae in freshwater and in aquatic snails with robust species-specific DNA assays will be required. Appropriate metrics, derived from observed prevalence(s) as compared with predetermined thresholds, could each provide a clearer insight into contamination- and exposure-related dynamics. Application could be twofold, first to certify areas currently free from schistosomiasis transmission or second to red-flag recalcitrant locations where extra effort or alternative interventions are needed.

Getting a GRiPP on everyday schistosomiasis: experience from Zimbabwe

Schistosomiasis, commonly known as bilharzia, is a parasitic disease prevalent in Africa, Asia and South America. The majority of the cases occur in Sub-Saharan Africa where schistosomiasis is a major public health problem impacting on child health and development as well as adult health when infections become chronic. Control of schistosomiasis is by treatment of infected people with the antihelminthic drug praziquantel. Current schistosome control programmes advocated by the World Health Assembly in 2001 are aimed at regular school-based integrated deworming strategies in order to reduce development of severe morbidity, promote school health and to improve cognitive potential of children. Several countries in Africa have now embarked on national scale deworming programmes treating millions of children exposed to schistosomiasis in endemic areas without prior diagnosis of infection through mass drug administration programmes. Implementing such control programmes requires a concerted effort between scientists, policy makers, health practitioners and several other stake holders and of course a receptive community. This paper considers the contributions to global schistosome control efforts made by research conducted in Zimbabwe and the historical context and developments leading to the national schistosomiasis control programme in Zimbabwe giving an example of Getting Research into Policy and Practice.

Mycobacteriosis-associated mortality in wild striped bass (Morone saxatilis) from Chesapeake Bay, U.S.A

The striped bass (Morone saxatilis) is an economically and ecologically important finfish species along the Atlantic seaboard of the United States. Recent stock assessments in Chesapeake Bay (U.S.A.) indicate that non-fishing mortality in striped bass has increased since 1999, concomitant with very high (>50%) prevalence of visceral and dermal disease caused by Mycobacterium spp. Current fishery assessment models do not differentiate between disease and other components of non-fishing mortality (e.g., senescence, predation); therefore, disease impact on the striped bass population has not been established. Specific measurement of mortality associated with mycobacteriosis in wild striped bass is complicated because the disease is chronic and mortality is cryptic. Epidemiological models have been developed to estimate disease-associated mortality from cross-sectional prevalence data and have recently been generalized to represent disease processes more realistically. Here, we used this generalized approach to demonstrate disease-associated mortality in striped bass from Chesapeake Bay. To our knowledge this is the first demonstration of cryptic mortality associated with a chronic infectious disease in a wild finfish. This finding has direct implications for management and stock assessment of striped bass, as it demonstrates population-level negative impacts of a chronic disease. Additionally, this research provides a framework by which disease-associated mortality may be specifically addressed within fisheries models for resource management.

THE FITTING OF GENERAL FORCE-OF-INFECTION MODELS TO WILDLIFE DISEASE PREVALENCE DATA

Researchers and wildlife managers increasingly find themselves in situations where they must deal with infectious wildlife diseases such as chronic wasting disease, brucellosis, tuberculosis, and West Nile virus. Managers are often charged with designing and implementing control strategies, and researchers often seek to determine factors that influence and control the disease process. All of these activities require the ability to measure some indication of a disease's foothold in a population and evaluate factors affecting that foothold. The most common type of data available to managers and researchers is apparent prevalence data. Apparent disease prevalence, the proportion of animals in a sample that are positive for the disease, might seem like a natural measure of disease's foothold, but several properties, in particular, its dependency on age structure and the biasing effects of disease-associated mortality, make it less than ideal. In quantitative epidemiology, the "force of infection," or infection hazard, is generally the preferred parameter for measuring a disease's foothold, and it can be viewed as the most appropriate way to "adjust" apparent prevalence for age structure. The typical ecology curriculum includes little exposure to quantitative epidemiological concepts such as cumulative incidence, apparent prevalence, and the force of infection. The goal of this paper is to present these basic epidemiological concepts and resulting models in an ecological context and to illustrate how they can be applied to understand and address basic epidemiological questions. We demonstrate a practical approach to solving the heretofore intractable problem of fitting general force-of-infection models to wildlife prevalence data using a generalized regression approach. We apply the procedures to Mycobacterium bovis (bovine tuberculosis) prevalence in bison (Bison bison) in Wood Buffalo National Park, Canada, and demonstrate strong age dependency in the force of infection as well as an increased mortality hazard in positive animals.